Duplication of items between bank and ESO+ bank

[vsrs_au]

I wrote an addon to (among other things) count the items in the account's bank, using the 2 bag type codes defined in the ESOUI API, i.e. BAG_BANK and BAG_SUBSCRIBER_BANK. The problem is that my code is finding various instances of the same items appearing in both shared inventories. My (partial) code is below:

function InventoryDump.DumpInventory(bagType)
	local iUsed = GetNumBagUsedSlots(bagType)
	local iTotal = GetBagSize(bagType)
	if bagType == BAG_VIRTUAL then
		CHAT_SYSTEM:AddMessage("InventoryDump:     used=" .. iUsed)
	else
		CHAT_SYSTEM:AddMessage("InventoryDump:     used=" .. iUsed .. "/" .. iTotal)
	end

	for _, data in pairs(SHARED_INVENTORY.bagCache[bagType]) do
		if data ~= nil then
			local itemLink = GetItemLink(bagType, data.slotIndex)
			local invCount, bankCount, cbCount = GetItemLinkStacks(itemLink)
			local count = 0
			if bagType == BAG_BANK or bagType == BAG_SUBSCRIBER_BANK then
				count = bankCount
			elseif bagType == BAG_VIRTUAL then
				count = cbCount
			elseif bagType == BAG_BACKPACK then
				count = invCount
			end
			InventoryDump.DumpItem(bagType, count, itemName)
		end
	end

	CHAT_SYSTEM:AddMessage("InventoryDump: inventory dumped.")
end

If I call the function first for BAG_BANK and then for BAG_SUBSCRIBER_BANK, the message output to the chat window correctly shows the used and used+free for the 2 shared inventories, e.g. at the moment it will show

InventoryDump:    used=165/240
InventoryDump:    used=58/240

When I use the bank UI in the game, it shows I have 223 items out of 480. So the overall counts seem to be correct. But when I examine the item names and counts written to the SavedVariables file, I find that some items are appearing in both these shared inventories (with the same stack count), and others only appear in 1 of these shared inventories. Does anyone know why?